I think that's your door bell. the chain latch is cool nice design on it.
the door probably had a latch where it is external to the door . I will look for a pic of what I mean. is there no hole fotr the striker cut?
I found this latch picture . I want one like this !
http://previews.123rf.com/images/aantip ... -Photo.jpgthis is the kind I was thinking of. ( link below) If it had one like this it might explain why there is a hole in the door but no hole for the latch. if you buy one make sure it is the right conf=iguration . I found a neat one and then found I couldn't use it because it is for a door that opens the other way so the latch ad the rounded latch part shaped the wrong way and it wasn't reversible. If it had that kind of latch you;d expect to see some screw holes int he door and beside the door whare it once was. the actual outside lock part , the part in the door can be a common modern one I think.
http://www.terapeak.com/worth/antique-d ... 730853250/I like htis latch because if I flip the lever down its locked from the outside but I can get out and if I flip it up it can't be opened from the outside even with the key. when I go out I have a deadbolt and if that is locked you can't open it from either side so it makes it a little harder for a crook to use that door to cart stuff out if he does break in some other way. the thing to be careful of is you dont' wan the situation where you lock yourself in and need the key to get out if there is ever a fire.
I dont' know if that made sense but I use the deadbolt or both, when no one is home and I use this latch if I feel like locking the door when I am home in case I need to get out in a hurry I still can. If you look at some utube videos on "lock bumping" it shows how easy it is to get almost any lock to unlock, and how to make special keys for lock bumping. but if you have two locks it becomes a lot more difficult to do that.
some of those latches are old cast iron and some are alunimum so if you get one online try not to get the newer alunimum kind if you can.
it looks like you are into some fun with the walls. !